1. Field of the Invention
The present invention relates generally to the field of hydrometallurgical extraction of precious metals from ores. More specifically, the present invention involves improvements to extraction of precious metals from ores by means of chlorinated brines, or brines containing hypochlorite salts.
2. Prior Art
Prior to introduction of the cyanide method in the late 1800s, gold ores were treated on an extensive scale by roasting and subsequent leaching with aqueous solutions of chlorine. The prior art also contains extensive references dealing with leaching of silver ores by chloride solutions. These processes are based on the well-known solubility of silver chloride in concentrated chloride solutions. Other prior art references of marginal relevance involve roasting of ores in the presence of chlorine to convert sulfides to chlorides.
U.S. Pat. No. 3,647,261 of Stenger, et al., is a general reference disclosing use of chlorine or alkali metal hypochlorite (e.g. NaOCl) in brine to solubilize silver for in situ leaching of ores. Other than claiming a desired range of oxidation potentials, Stenger provides little guidance concerning optimum pH, or concentrations of chloride, hypochlorite, or free chlorine for leaching precious metals. Precipitation of silver and copper from the pregnant lixiviant by hydrogen sulfide or sulfide salts is discussed (col. 4, lines 28-33).
U.S. Pat. No. 4,342,592 of Lamb is another general reference disclosing use of a hypochlorite/brine lixiviant with a pH from 8 to 13 to leach precious metal values from ores. Silver is recovered from the pregnant lixiviant by substitution with a more electropositive metal, such as zinc or iron (col. 3, lines 34-44).
In contrast, the present invention involves a number of improvements and refinements to the basic process not taught by any of the prior art references. In particular, the lixiviant disclosed herein has an essentially neutral pH from 5 to 8, and a hypochlorite concentration substantially less than any taught by the prior art as being effective. This results in substantial savings both in initial material costs, and in minimizing loss of hypochlorite in processing.
Hypochlorite loss may become a severe problem with in situ leaching or in large-scale heap leaching where a long period of time is required for the lixiviant to percolate through the ore. None of the prior art references address this problem. The present invention greatly reduces these losses by including cyanuric acid in the lixiviant to inhibit decomposition of the hypochlorite ions. Cyanuric acid and related salts are widely used to stabilize chlorine solutions used in bleaches, cleaning compounds, and swimming pools. However, the prior art references in these fields, such as U.S. Pat. No. 2,988,471 of Fuchs, et al. teach away from the present invention by observing that cyanuric acid inhibits chemical interaction between chlorine in solution and metal fixtures and pipes.
The present invention also differs from the prior art in adding an intermediate step to eliminate any remaining hypochlorite ions from the pregnant lixiviant prior to recovery of the metal values. Use of relatively inexpensive reducing agents or hydrogen peroxide for this purpose greatly simplifies the chemistry and reduces the cost of subsequent recovery of metal values from the pregnant lixiviant. One particularly promising result of this intermediate step is that simple carbon, such as charcoal or activated carbon, can then be used to reduce the metal value from solution. In the absence of this intermediate step, the carbon would be oxidized by the hypochlorite ions remaining in the lixiviant causing an economic loss of carbon. Further, hypochlorite remaining in the lixiviant greatly hinders the rate of deposition of precious metals. Alternatively, if the ore contains sulfides, a small portion of finely ground ore can be added to the pregnant lixiviant to reduce the metal values from solution. Neither alternative is taught nor suggested by the prior art.